Power converters such as switch-mode voltage regulators are widely used in various electronic devices. In the presently existing switch-mode voltage regulators, a high voltage signal may be needed to drive the switches. So a bootstrap circuit will be applied to provide the high voltage signal.
FIG. 1 illustrates schematically a voltage converter 50. As shown in FIG. 1, the voltage converter 50 comprises a switching circuit 51 and a control circuit 52.
The switching circuit 51 may comprise a high side switch 11, a low side switch 12, an inductor (L), a capacitor (C) and a load (R). The high side switch 11 and the low side switch 12 have a source, a drain and a gate respectively. The high side switch 11 and the low side switch 12 may comprise a power switching device, such as a Metal Oxide Semiconductor Field Effect Transistor (MOSFET), a Junction Field Effect Transistor (JFET) etc. The drain of the high side switch 11 is coupled to an input terminal (IN) of the voltage converter 50 for receiving an input voltage (VIN). The source of the high side switch 11 is coupled to the drain of the low side switch 12 so as to constitute a common connection node (SW). The source of the low side switch 12 is connected to a logic ground. The inductor (L) is coupled between the common connection node (SW) and an output terminal (OUT) of the voltage converter 50. The capacitor (C) and the load (R) are connected in parallel between the output terminal (OUT) of the voltage converter 50 and the logic ground.
The control circuit 52 is coupled to the output terminal (OUT) of the voltage converter 50 for receiving a feedback signal FB representing an output voltage signal (VOUT) of the voltage converter 50, and configured to generate a first control signal (SH) and a second control signal (SL) to switch the high side switch 11 and the low side switch 12 on and off in a complementary manner. In one embodiment, the control circuit 52 may comprise a Pulse-Width Modulation (PWM) control circuit, wherein the PWM control circuit is configured to regulate the output voltage signal (VOUT) by providing a plurality of square pulse signals with different duty cycle.
In the embodiment as shown in FIG. 1, the control circuit 52 may further comprise a driver configured to drive the high side switch 11 and the low side switch 12. During positive energy storage on the inductor (L), the driver may pull up the voltage of the gate of the high side switch 11. In such application, in order to make the high side switch 11 to be fully turned on (i.e. to make the high side switch 11 to operate in saturation region in which the switch 11 has a quite small on resistance), a voltage applied between the gate of the high side switch 11 and a terminal connected to the common connection node SW of the high side switch 11 must be large enough, at least larger than a conduction threshold voltage of the switch 11. However, when the high side switch 11 is on, the voltage at the common connection node (SW) can reach the input voltage (VIN), and thus a voltage higher than the input voltage (VIN) must be provided to the gate of the high side switch 11 so as to turn it on completely.
Therefore, in order to generate a voltage higher than the input voltage (VIN), the voltage converter 50 generally further comprises a bootstrap circuit 53. The bootstrap circuit 53 is configured to provide a bootstrap voltage (VBST) referenced to the common connection node (SW). The bootstrap voltage (VBST) can be used to enhance the driving capability of the first control signal (SH) provided to the gate of the high side switch 11, so that the first control signal (SH) can drive the high side switch 11 to turn on and off in good condition.
In the example of FIG. 1, the bootstrap circuit 53 is illustrated to comprise a diode (DB) and a bootstrap capacitor (CB) connected in series between a bootstrap supply terminal and the common connection node (SW), wherein a cathode of the diode (DB) is connected to the bootstrap supply terminal to receive a bootstrap supply voltage (VB), an anode of the diode (DB) is connected to a first terminal of the bootstrap capacitor (CB) to constitute a common connection node (BST), and a second terminal of the bootstrap capacitor (CB) is connected to the common connection node (SW). The operating principles of the bootstrap circuit 53 can be easily understood by one of ordinary skilled in the art. When the high side switch 11 is turned off and the low side switch 12 is turned on, the voltage of the common connection node (SW) is equal to the ground potential and the bootstrap capacitor (CB) is charged by the bootstrap supply voltage (VB) till the voltage across the bootstrap capacitor (CB) reaches the bootstrap voltage (VBST). When the high side switch 12 is turned on and the low side switch 11 is turned off, the input voltage (VIN) of the voltage converter 50 is transmitted to the common connection node (SW), i.e. the voltage at the second terminal of the bootstrap capacitor (CB) is pulled up to the input voltage (VIN). Thus, the voltage at the common connection node (BST) is raised to a voltage higher than the input voltage (VIN), substantially equal to the input voltage (VIN) plus the bootstrap voltage (VBST). Thus, a voltage higher than the input voltage (VIN) can be obtained in the step-down voltage converter 50. Meanwhile, the diode (DB) is reversely biased and is thus turned off so as to protect the bootstrap supply voltage source (VB) from being damaged by the relatively higher input voltage (VIN).
In view of the above, it can be understood that the bootstrap capacitor (CB) can be charged to refresh the bootstrap voltage (VBST) by pulling down the second terminal of the bootstrap capacitor (CB) to ground potential when the low side switch 12 is turned on. However, in certain circumstances, the bootstrap capacitor (CB) may not have enough charge stored and may not be charged/recharged in time, resulting in the bootstrap voltage (VBST) to be decreased, which may cause the first control signal (SH) unable to drive the high side switch 11 on and off properly. Consequently, the voltage converter 50 can not be operated properly. Or in another situation, the bootstrap voltage (VBST) may drops to a default lock-out threshold value resulting in a lock of the voltage converter 50, which may also cause the voltage converter 50 operating improperly.
For example, when the output voltage signal (VOUT) is approximately close to the input voltage signal (VIN), the high side switch 11 has to operate with a quite high duty cycle, even in a 100% duty cycle. Therefore, the low side switch 12 may have a very short conduction time or hardly have chance to turn on resulting in the bootstrap capacitor (CB) unable to be charged to a desired value.
In another condition, for example, when the voltage converter 50 operates in light load or no load condition, the control circuit 52 is configured to reduce the conduction time and/or the switching frequency of the high side switch 11 and the low side switch 12 to improve the conversion efficiency of the voltage converter 50, which may lead to the bootstrap capacitor (CB) not being able to be charged/recharged in time since the on time of the low side switch 12 is too short or there is no switching in a relatively long time.
Therefore, all the conditions mentioned above can result in drops of the bootstrap voltage (VBST), which in turn causes failure in driving the high side switch 11 on and off properly. Thus, the voltage converter 50 operates in an abnormal condition. In such circumstances, only when the output voltage signal (VOUT) drops low enough after several operation cycles, can the bootstrap capacitor (CB) have the opportunity to be charged to refresh the bootstrap voltage (VBST). However, during such processes, a plurality of large spikes may occur in the output voltage signal (VOUT), which may damage the voltage converter 50 and the load (R). Thus, it is undesirable.
A need therefore exists for solving the problem of refreshing the bootstrap voltage (VBST) timely in voltage converters.